[llvm-bugs] [Bug 45285] New: -pedantic complains about multi-line comments // ... \

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https://bugs.llvm.org/show_bug.cgi?id=45285

            Bug ID: 45285
           Summary: -pedantic complains about multi-line comments // ... \
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: matthias.andree at gmx.de
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

Greetings,

clang (tried multiple versions) warns about legal multi-line //-style comments,
as part of -Wcomment which is default-enabled.

Minimal code (to try with, for instance, -std=c99, =c11, =c++11), save as
/tmp/try.c, and them compile with:

clang -pedantic-errors -std=c11 -O -c /tmp/try.c
clang -pedantic-errors -std=c++11 -O -c /tmp/try.c

// \
%
void foo(void) {}

Yields:

/tmp/try.c:1:4: error: multi-line // comment [-Werror,-Wcomment]
// \
   ^
1 error generated.

Expected output:
none, and compilation succeeds.


I am mentining these three languages because I've read up on the latest draft
specs. All three language specifications (their final drafts at any rate)
concur about phases of compilation that:

* phase 2 in a single-pass motion removes a single backslash followed by line
break

* phase 3 replaces comments by one space

And if I understand correctly, this is not a language extension, so -pedantic
should not complain. The code above should be equivalent to (line end marker
¶), after phase 3:

 ¶
void foo(void) {}¶


Looking at GCC, 9.2.0 and 10 as of 20200315 accept the code above without
diagnostics or warnings.

Please check if LLVM should disable the warning under -pedantic.

I was testing on FreeBSD 12.1-RELEASE-p3, amd64 (aka x86_64),

llvm80-8.0.1_3                 LLVM and Clang
llvm90-9.0.1                   LLVM and Clang
llvm10-10.0.0.r4               LLVM and Clang
llvm-devel-11.0.d20200117      LLVM and Clang

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